Clinical and experimental data support a pathogenic role for MHC-directed alloantibodies (alloAb) in both acute and chronic rejection after solid organ and cell transplantation. The absence of monitoring tools to detect and specific immunotherapies to suppress humoral alloimmunity presents significant clinical challenges. Cellular transplants are likely at higher risk for rejection and graft loss due to humoral immunity. Despite the importance of humoral alloimmunity in clinical transplant, there is relatively little known about the regulation of isotypes, quantity, specificity and pathogenicity of alloAb after transplant. The overall focus of this research project is to understand fundamental cellular and molecular mechanisms regulating alloAb production in the presence or absence of conventional immunosuppression. Our manuscript in press provides first evidence of the pivotal role that IFN?+CD8+ T cells play in inhibiting the magnitude of post transplant alloAb production. Our studies to elucidate this novel CD8-dependent mechanism of post transplant alloAb regulation represents a paradigm shift necessary to identify new cellular and molecular therapeutic targets. In Aim 1, we will investigate the role of innate and adaptive immune cell subsets and cytokines/cytokine receptors critical to CD8-mediated inhibition of post transplant alloAb production. In Aim 2 we will identify cytotoxic effector and MHC molecules necessary for CD8-mediated killing of alloprimed B cells and identify gene and protein expression profiles associated with alloprimed B cells in high and low alloAb producers. In Aim 3 we will determine how mTOR and CN inhibition differentially suppress posttransplant alloAb production in vivo.